Earth shattered heat records in 2023 and 2024: is global warming speeding up?

Nature examines whether the temperature spike is a blip or an enduring — and concerning — trend

Earth’s temperature has been climbing for decades.Credit: Mark J. Terrill/AP/Alamy

Earth’s temperature has surged in the past two years, and climate scientists will soon announce that it hit a milestone in 2024: rising more than 1.5 °C above pre-industrial levels. But is this sudden spike just a blip in the climate data, or an early indicator that the planet is heating up at a faster pace than researchers thought?

That question has been at the centre of numerous studies, as well as a session at last month’s meeting of the American Geophysical Union (AGU) in Washington DC. Some scientists argue that the spike can be mostly explained by two factors. One is the El Niño event that began in mid-2023 — a natural weather pattern in which warm water pools in the eastern tropical Pacific Ocean, often leading to hotter temperatures and more-turbulent weather. The other is a reduction over the past few years in air pollution, which can cool the planet by reflecting sunlight back into space and seeding low-lying clouds. Yet neither explanation fully accounts for the temperature surge, other researchers say.

Clouds clearly played a part, according to a study in Science, published in December just before the AGU meeting¹. A research team identified a reduction in low-lying cloud cover across parts of the Northern Hemisphere and the tropics that, combined with the El Niño, was large enough to explain the temperature spike in 2023. But the cause of this decrease — and whether it can be chalked up to normal climate variations — remains a mystery, the authors say. Decreased air pollution alone doesn’t seem to explain it. They suggest that global warming itself could be causing some reduction in cloud coverage, creating a feedback loop that could accelerate the rate of climate change for decades to come.

“I would be very careful about saying this is clear evidence [of acceleration], but there might be something going on,” says co-author Helge Goessling, a climate physicist at the Alfred Wegener Institute in Bremerhaven, Germany.

Global temperature spikes have happened before. Why are scientists so worried about this one?

One reason is that global temperatures were off-the-charts hot in 2023, with an average 1.45 °C of warming above the pre-industrial baseline (see ‘Temperature surge’), shattering previous records. This level of warming is outside the range of what scientists expected on the basis of previous trends and modelling.

Source: Copernicus Climate Change Service/European Centre for Medium-Range Weather Forecasts

Another reason is that last year was also much warmer than expected. Scientists projected that early 2024 would be hot owing to the El Niño. But they also anticipated that temperatures would fall after the weather pattern subsided and conditions in the equatorial Pacific returned to normal last June.

“That didn’t happen,” says Zeke Hausfather, a climate scientist at Berkeley Earth, a non-profit organization in California that tracks global temperatures. Instead, global temperatures remained elevated, shattering more records and likely making last year the warmest on record by a sizeable margin. “All of us who made projections at the start of the year underestimated just how warm 2024 would be.”

Is there evidence of acceleration?

Some say that the massive temperature spike might end up being a blip in the climate data, owing in large part to new regulations covering air-pollution from ocean-going ships. In 2020, the United Nations International Maritime Organisation implemented a rule requiring an 80% reduction in sulfur emissions from ships sailing in international waters. One analysis of satellite imagery, published in August, suggests that there has been a clear reduction in ship tracks², which are formed when sulfur-containing pollution particles seed low-lying mists. The changes seem to correlate with the broader reduction in cloud cover pinpointed by Goessling’s team.

“It’s almost a smoking gun,” says Andrew Gettelman, a climate scientist at the Pacific Northwest National Laboratory in Richland, Washington. If true, Gettelman says, this would indicate that the recent temperature spike could be a one-time phenomenon driven by short-term changes in pollution and a powerful El Niño.